CCM3 is one of mutated genes responsible for the human CCM disease, a pathological condition that affects the vasculature of the central nervous system and results in stroke, seizure and cerebral hemorrhage with a high prevalence. CCM consists of dilated and multiple capillary channels formed by a single layer of endothelium, lacking all other normal vessel wall elements. Patients with inherited autosomal dominant CCM carry loss of function mutations in one of three genes: CCM1, CCM2 and CCM3. Deletion in one of the three Ccm genes in vascular EC induces CCM lesions in mice. However, it is unknown why both humans and mice with CCM3 loss exhibit more severe phenotype than those with loss of CCM1 or CCM2. Our unexpected discovery of the involvement of CCM3 in EC exocytosis, prompt us to hypothesize that alteration in CCM3-regulated EC exocytosis contributes to the pathogenesis of the CCM disease. We propose the following two specific aims: 1) To determine the role of CCM3-regulated brain EC exocytosis in CCM disease phenotypes. We will establish mouse CCM models even closer to human CCM disease by creating brain EC-specific Ccm3 deletion, and determine therapeutic effects of Angpt2 neutralization antibodies in the new CCM3 mouse models. 2) To explore crosstalk of CCM3-mediated EC exocytosis with other pathways implicated in CCM formation. We will test if inhibition of exocytosis in ECs blocks RhoA-dependent EC stress fiber formation, TGF-?/Smad/BMP-mediated EndMT signaling, and MEKK3-ERK5-KLF4-mediated matrix remodeling. Conversely, test if gain- or loss-of-function of RhoA, TGF-? and MEKK3-ERK5-KLF4 signaling regulate EC exocytosis. In summary, the complementary approaches using genetic, cell biological and imaging analyses will facilitate our understanding of the molecular mechanisms and pathogenesis involved in acquisition of cerebral cavernous malformations, and help in defining new and more effective therapies. Our findings should benefit the general understanding of the regulatory mechanisms of exocytosis, which also occurs in other cardiovascular cells and ECs of other cardiovascular organs such as heart, lung and aortae. Therefore, our present study is of broad significance in cardiovascular research.